Mixing Cast Iron with radiant panels, matching temps

Robert_H

We are starting to finalize plans for our heating system which will be mixing cast iron rads with hydronic ceiling panels (1 panel now, three more upstairs next year).



I like the simplicity  of TRVs on the rads and the ceiling panel with all running on the outdoor reset. The more I read, the more this seems to be the way to go. Obviously with the ceiling panel on a separate zone.  



I’ve been focusing on keeping the water temp requirement low to modulate as much as possible. And trying to keep the radiators and the ceiling panel operating at the same temp thinking the closer I can do that, the more energy efficient the system will be.  



The radiators are oversized and I have worked out the output at water temps down to 110 deg.  Assuming max 140 degrees for the ceiling panel,  I can arrange the rads to meet the design heat loss in their spaces at 140 water temp. Actually all exceed the heat loss by a few hundred degrees at 140.



Any comments? Am I going in the right direction on this or smoking something?



My contractor and supplier are leaning toward the Lochinvar Cadet.  The supplier is running it by the rep. (1300 sqft house with basement. Approximately 40K heat loss.)



Also, I am putting a towel warmer rad in the bathroom.  The kitchen has no place for a cast iron rad. However, I have space in the room  to put a large vertical panel radiator.



Will using these different type radiators, Cast rads , panel rad, towel rad, and radiant ceiling, with different mass and response times negate my efforts to get them all at the same design water temp?



Thanks a bunch!

Robert

Robert_H

Dont know

if this makes any difference but we already have a gas tank-less water heater. So the Boiler will not provide DHW.

Zman

I'll smoke some...

I like it.

If you are confident in your design temp, just size your panels to match the heat loss at that temp. Uponer has charts for all that. http://www.uponor-usa.com/misc/cdam-request-page/default.aspx

The section you want is toward the end.

If you cannot get the tubing tight enough you could either do a wall panel or as you suggest, run the radiators a little hot.

Carl

Robert_H

Thanks

for your help Zman

Robert_H

Radiant Ceiling Temp control

As previously stated I wanted to run a simple circuit with one Delta T circulator on the secondary circuit with the boiler on outdoor reset, TRVs on the Rads and some type of thermostatic valve on the Ceiling Panel Header.  Something like in the first diagram.

However the designer at the supply house is concerned about the water going to the radiant ceiling getting too high. Although the rads are oversized to run at 140 degrees at ODT he is concerned that if the boiler puts out higher than 140 degrees it could damage the ceiling.   He first suggested using a mixing circuit as in the second diagram.  



I didn’t want to run an additional pump. But if it has to be it get over it…  He made it clear that his use of radiant is limited and is going to check with the manufacture of the boiler (Viessmann) and Roth on other options. We also agreed that I would check back here.

Based on experiences what’s the best way to handle this?

SWEI

Matching temps

Would be simplest.  Ceilings and walls can tolerate higher temp than floors.



The B2HA can control a motorized mixing valve and run two separate ODR curves if you need to.

Gordy

120 max

supply On the ceiling panels.



I can't imagine needing more than that ever



I top out at 115 on a design day -10 with my radiant ceilings. But that depends on your ceiling panel detail. Plates, centers, sf of area covered etc..



So that means a mixing valve for the ceiling panel supply.



You could control the ceiling panels with TRVs also.

Robert_H

SWEI

"Matching Temps Would be simplest.  Ceilings and walls can tolerate higher temp than floors."



What would you use as the highest temp for a ceiling? the reason Im asking is that we talked about running the the ceiling on a TRV (no mixing) and using an aqua-stat to shut off the supply if the supply temp got to high.



"The B2HA can control a motorized mixing valve and run two separate ODR curves if you need to."



Nice, I think that means I could run the boiler on one ODR and then a mixing valve for the ceiling on the other.

SWEI

Ceiling temp limits

depend on materials.  Drywall has an official limit of 120ºF.  Plaster or cement board can handle as much as PEX will, though it's better to add more tubing and lower the temp if only for evenness.



And yes, the B2HA will run the whole show nicely with two different temps and complete control over both curves.  Lochinvar can do this with an add-on box.  Most others qould require a separate control for the mixing valve (or a Taco iSeries-R valve.)

Robert_H

120 it is

As we are using drywall in the end, and thats fits with Gordy's reply as well.



I should have asked this before but if we go with a mixing valve requiring a supporting pump,

Should it be on continuous circ with the rest of the system?

should it be fixed or variable speed?

SWEI

Pump choices

Outdoor reset control really plays best with constant circulation.



Traditional zoning (using zone pumps or on/off zone valves) does not pair well with ODR, unless you crank up the curve to the point where the zone controls have some authority, which burns more fuel and delivers less comfort.  Standard on/off zone valves and thermostats are used only as high limit controls for spaces that have external gains (south facing glass, wood stove, etc.) or for infrequently-occupied spaces that can be warmed up a day in advance of their use.  To fully zone a mod/con boiler (or any ODR-controlled system) we use  TRVs or motorized proportional control valves.  If you have zone valves in your system, a ∆P ECM circulator works well.  If there are no valves, then a ∆T circ is better.  Even if you run an ECM circ at a fixed speed, it will draw a fraction of the power of a standard pump. 



With those old radiators, be absolutely certain you clean & flush the system well.  A magnetic dirt separator is also a good idea for those.

Robert_H

Thanks Gordy and SWEI!

I had a long discussion with Rob today and we finalized the plans.

Thanks for your help!

SWEI

One small favor in return?

Let us know how it all works out.



Best of luck.